1. Field of the Invention
The present invention relates to an optical pickup device for an optical disc capable of information recording and reproducing, and more particularly to an optical pickup device suitable for stable light emission of a semiconductor laser in reproducing information recorded on an optical disc.
2. Related Background Art
FIG. 5 shows a total spectral reflectance characteristic of antireflection thin films used in an optical system of a conventional optical pickup device.
In an optical pickup device having a function of recording information on an optical disc and reproducing recorded information, an optical system providing an optical path for a laser beam includes, as shown in FIG. 3, a semiconductor laser 1 for emitting a laser beam, a collimator lens 2, a quarter wavelength plate 3, a diffraction grating 4, two beam shaping prisms 5, a beam splitter 6, a mirror 7, and an objective lens 8 disposed in this order. A laser beam passed through this optical system is applied to an optical disc 8 in the form of a laser beam spot.
In an optical pickup device dedicated to only information reproduction, an optical system providing an optical path for a laser beam includes, as shown in FIG. 4, a semiconductor laser 1, a collimator lens 2, a diffraction grating 4, a beam splitter 6, a mirror 7, and an objective lens 8 disposed in this order. A laser beam passed through this optical system is applied to an optical disc 8 in the form of a laser beam spot.
As compared with the optical system for an optical pickup device dedicated only to information reproduction, because the optical system for both information recording and reproducing requires a higher output power when recording information than when reproducing information, the structure of this optical system becomes more complicate in order to effectively use the light emission amount of the semiconductor laser 1.
In order to efficiently guide a laser beam to the optical disc 9 when recording information, antireflection thin films have been generally formed at laser beam transmission surfaces of each optical component of the optical pickup device for both information recording and reproducing. That is to say, the antireflection thin films are formed at transmission surfaces of the collimator lens 2, quarter wavelength plate 3, diffraction grating 4, two beam shaping prisms 5, beam splitter 6, mirror 7, and lens 8.
As shown in FIG. 5, the spectral reflectance characteristic of antireflection thin films used in a conventional optical system has generally a flat distribution of reflectances R in the range of light emission wavelengths covered by the semiconductor laser 1 from the peak wavelength .lambda. (L) at a low output power to the peak wavelength .lambda. (H) at a high output power.
In order for a photodetector (not shown) to stably detect a light beam reproduced by birefringence at the optical disc, films having substantially the equal transmission and reflectance characteristics both for P and S polarizations have been formed on surfaces of the beam splitter 6. In this way, a constant amount of light is reflected to the photodetector without being affected by the polarization characteristic of a light beam reflected from the optical disc 9.
With the conventional optical pickup device, however, the reflectance of a beam splitter has been unable to set too high, because the transmission of a laser beam to an optical disc required low reflectance in order to obtain a large amount of laser beam light when recording information.
Since the laser beam transmission is maintained at a sufficiently high level, light reflected from the optical disc passes more through the beam splitter and returns to the light emission surface of the semiconductor laser. Therefore, light emission at the semiconductor laser operating at a low output power when reproducing information may fluctuate.